1. Field of the Invention
The present invention relates to a coordinate input apparatus suitably used as an input apparatus of information processors such as a personal computer and a word processor.
2. Description of Related Art
A coordinate input apparatus called a tablet has been used as an input apparatus for inputting hand-writing letters and graphics to electronic equipment such as a personal computer and a word processor. The coordinate input apparatus is also called a touch panel. The coordinate input apparatus has a pointing area which is a two-dimensional plane. When the user presses down the pointing area continuously, similar to drawing a line on a paper with a pen, the coordinate input apparatus outputs values of coordinates which correspond to each of a plurality of points on the line in order of the passed points.
The pointing area of such coordinate input apparatus is constructed often by a translucent member for example. This member is used by laying it on top of a visual display area of a display unit such as a cathode ray tube or liquid crystal display. At this time, the display unit changes pixels which correspond to the coordinate output from the coordinate input apparatus, i.e. the pixels right under the position of the area which has been pressed down by the user, from white display to black display. Thereby, a curve approximated by the input points thus inputted and broken lines connecting the input points is displayed on the display unit.
FIG. 18 is a block diagram showing an electrical structure of a resistor film type coordinate input apparatus. Two resistor films having the same size with a pointing area 3 for example are disposed while leaving a space therebetween in the resistor film type coordinate input apparatus. Two pairs of electrodes 4, 5; 6, 7 are attached to the pair of resistor films in the directions vertical to each other. For example, the electrodes 4, 5 detect the coordinate of a position in the X direction indicated by an arrow 17 in the pointing area 3 and the electrodes 6, 7 detect the coordinate of the position in the Y direction indicated by an arrow 18 in the pointing area 3.
One pair of electrodes attached in one direction of the pointing area are used as scanning electrodes so as to apply a gradient voltage and the other pair of electrodes are used as detection electrodes. When the user presses down one point in the pointing area by a pen or by a finger, the resistor films contact at that point and the voltage between the electrodes in one direction is divided. The values of the voltage thus divided are detected by the electrodes in the other direction. A ratio of division of the gradient voltage obtained from this voltage value is equal to a ratio of a distance between the electrodes divided by the input point. Accordingly, the value of coordinate of the input point in one direction can be obtained from the values of voltage thus divided. The two-dimensional coordinate of the input point within the pointing area can be detected by using the above-mentioned two pairs of electrodes while switching them alternately in a short time. The resistor film type coordinate input apparatus detects the values of the coordinate of the input point by checking variations of the values of voltage among each of those electrodes 4, 5 as well as 6, 7.
The values of voltage detected by the electrodes 4, 5 as well as 6, 7 are supplied to analog/digital converter circuits 9 and 10 (hereinafter abbreviated as "A/D converter circuit" in the figure), respectively. The analog/digital converter circuits 9 and 10 convert analog outputs from the electrodes 4, 5; 6, 7 into digital outputs for sampling per predetermined unit time measured by a timer 11. The outputs from the analog/digital converter circuits 9 and 10 are stored in an X coordinate buffer 14 and a Y coordinate buffer 15 in a buffer 13 as the X coordinate and the Y coordinate of the input point output from the pointing area 3. The above-mentioned electronic equipment reads the X and Y coordinates of the input point from the respective buffer areas 14 and 15 in the buffer 13 to use in the processing thereafter.
In the coordinate input apparatus described above, the input point is inputted by the user who presses down the pointing area by a finger or by holding a pointing member such as a pen, with a similar operation to drawing a line on a paper with a pen. The input point pointed by the user is the point pressed down by the pointing member. At this time, if the hand holding the pointing member erroneously touches the pointing area and presses down the resistor film, sometimes the coordinate of the position which the hand touched is output erroneously as a coordinate of the input point. Further, if a degree of pressure for pressing down the pointing area by the pointing member is less than the pressure necessary for contacting the pair of resistor films, the values of the coordinate which corresponds to the input point may not be output.
Still more, when the coordinate input apparatus described above is used as an input apparatus of an electronic equipment, the peripheral edge portion of the pointing area is often overlapped with a case of the electronic equipment. When a portion of the case which overlaps with the peripheral edge portion of the pointing area is held strongly while using the electronic equipment, the case may be pressed and deformed, thus pressing down the pointing area. Then, the coordinate of the position pressed at this time may be erroneously output as a coordinate of an input point.
The input coordinate inputted through the pointing area is output as an electrical signal for example. Accordingly, there may be case when noise is superimposed to the electrical signal during the time from when it is output from the pointing area 3 till stored in the buffer areas 14 and 15. Thereby, the values of the coordinate of the input point stored in the buffer areas 14 and 15 may deviate from the actual values by the noise.
Thus, the erroneous inputs by which the values of coordinate of the input point inputted from the pointing area 3 are erroneously output occur in the coordinate input apparatus. Then, the applicant of the present invention has proposed a display integrated tablet device which has been disclosed in Japanese Unexamined Patent Publication JP-A 6-295219 as a coordinate input apparatus which can eliminate the erroneous inputs described above. This tablet device enables to input coordinates by utilizing a constitution of a display panel such as a liquid crystal display. The display panel is constituted such that first and second electrode groups in which thin strips of electrodes are arrayed in parallel are disposed so as to face each other so that the longitudinal directions of the electrodes cross at right angles each other while leaving a space therebetween.
When a coordinate is to be inputted, scan voltage is applied alternately to the electrode groups to scan each electrode group sequentially. The user draws or contacts a detection pen near to/with a desirable position on the surface of the display panel. At this time, induced voltage is generated by floating capacity between the detection pen and the electrodes to which the scan voltage has been applied. The tablet device discriminates the input coordinate based on the variation of the induced voltage and timing when the voltage is applied to each electrode. At this time, the tablet device binalizes an output, i.e. the variation of the induced voltage of the detection pen, by a comparator to count a number of pulses. When the number of pulses thus counted is greater than an actual number of times of electrode scan, it is considered that there has been an erroneous input.
There has been disclosed a touch operating input apparatus which can eliminate an erroneous input caused by a structure peculiar to the tablet in Japanese Unexamined Patent Publication JP-A 4-57117. According to the input apparatus of the present publication, an optical touch panel for example is disposed on a display while overlaying a contact type touch panel. The user operates the two kinds of panels in the same time. The input apparatus discriminates that the input is correct only when the same coordinate is inputted on the two kinds of panels, respectively.
For example, the optical touch panel causes an erroneous input when a flying object such as an insect or foreign material infiltrates into the pointing area. The contact type touch panel causes an erroneous input when the structural member of the pointing area has a flaw or deformation. The contact type touch panel will cause no erroneous input even if a flying object infiltrates into the pointing area. Similarly, the optical touch panel will cause no erroneous input even if the structural member of the pointing area gets a flaw. Thus, the cause of the erroneous input of the touch panel is different depending on the structure thereof. Accordingly, even if the cause of the erroneous input described above occurs, no erroneous input will occurs on one touch panel, though an erroneous input occurs on the other touch panel.
Accordingly, it is possible to eliminate the erroneous input based on the cause peculiar to each individual touch panel by determining that the input is correct only when the coordinates which correspond to the same position are inputted respectively from the two overlaid panels. However, because the two kinds of coordinate input apparatuses are used in the touch operating input device described above, the structure is complicated. Accordingly, the size of the device becomes large, increasing the manufacturing cost thereof.
Further, a coordinate reader which calculates a position indicated by a coordinate indicator within a tablet and which allows an erroneous input to be eliminated has been disclosed in Japanese Unexamined Patent Publication JP-A 6-4203. This coordinate reader detects the coordinate based on induced current which is generated within the tablet by electrical coupling which is generated between the tablet and the coordinate indicator when they are drawn closer or are contacted. At this time, among variations of voltage caused within the tablet, variation caused by external noise is instantaneous and variation in inputting the coordinate continues for a certain period of time. Accordingly, when the variation of voltage occurs, the duration is counted by a timer to determine that the input coordinate is correct only when the variation of voltage continues more than a predetermined time set in advance. However, it is difficult to determine whether the coordinate is correct or not before a time more than the predetermined time passes since when the coordinate has been inputted.
A coordinate input apparatus which detects an input coordinate by using a capacitive coupling method for an X-Y matrix display and which allows the erroneous inputs to be eliminated has been disclosed in Japanese Unexamined Patent Publication JP-A 7-64704. When an input pen is abutted with the display in the coordinate input apparatus, a capacitive coupling occurs between the pen and the display. Thereby, scan voltage which has been applied to a display electrode of the display is also applied to an electrical circuit of the pen, thus causing a change in the output of the pen. The input coordinate is detected by specifying the display electrode which has been scanned when the change in the voltage has occurred.
A switch which turns off only when the pen is abutted with the display is provided at the edge of the pen. When the edge of the pen is not abutted on the display and the switch is on, impedance of the pen is dropped. Thereby, noise which mixes in when the pen is not abutting with the display is reduced. This method of eliminating the noise of the coordinate input apparatus is effective only for the coordinate input apparatus constructed as described above.
A coordinate detector and a coordinate detecting method which allow the influence of noise to be eliminated are disclosed in Japanese Unexamined Patent Publication JP-A 5-324163. A coordinate is inputted by pressing a resistor film sheet, i.e. an pointing area, by pen input means in this coordinate detector. When the resistor film sheet is pressed by the pen input means, values of coordinate at that time are sampled by a plurality of times to average the values of the sampling data. This average value is compared with each sampling data, the sampling data which deviates from the average value more than a predetermined deviation is excluded and only values of the remaining sampling data are averaged again. An average value of the remaining data is considered as the value of coordinate which has been inputted. The noise eliminated by this method is what is added on a power line of a power source which the tablet applies at the both ends of the resistor film sheet. Accordingly, it is different from what eliminates the erroneous input of the tablet.
Further, an information processor which reduces errors of specification of coordinate or area is disclosed in Japanese Unexamined Patent Publication JP-A 6-342336. Input means of this information processor is what inputs coordinates by pressing down by pointing means such as a finger or a pen. At this time, the pointing means validates the input coordinate only when pressure for pressing down the input means exceeds a predetermined value.
The input means of the information processor described above determines whether the coordinate inputted at this time is correct or not in correspondence to the pressure when the pointing means presses down the input means. Accordingly, if the pressure exceeds the predetermined value, it is difficult to eliminate the input coordinate even if it is actually erroneous. For example, when a line is to be drawn on the input means by the pointing means, an input coordinate is considered to be valid if the pressure in inputting it exceeds the predetermined value even if the input is made at the position separated from the position which is assumed from the direction and speed of the advancement of the pointing means.